Doors are essentially walls that can be toggled to be opened or closed. They were added in the Community Experience update. There are five tiers of doors: wooden, stone, golden, diamond, and amethyst. Doors can only be used by the owner and his teammates (with a totem).
A common problem with having multiple doors is that on death players lose their structures--forcing the members to make a new one or patch the hole. Try to have as little doors as possible - having multiple to one person not only increases the need to fix it when they're gone, but costs significantly more.
` Another problem may arise when other players intentionally block doors with walls / spikes to keep you from going out. Make sure to have the equipment to be able to destroy any structure. (Any tool will work for wooden, a hammer or pickaxe is needed for stone+ structures. All structures can be slowly broken by Amethyst tools).
All tiers of door require a workbench to craft. Like all other structures, a door may be placed only once every 0.8 seconds.